The present invention relates to microwave semiconductor devices, i.e. to semiconductor active devices capable of switching at frequencies above one GHz.
The present invention more particularly relates to monlithic microwave integrated circuits (MMICs), i.e. to integrated circuits containing many active devices and switching at a clock rate above one GHz.
The present invention also particularly relates to power microwave devices, i.e. to microwave transistors capable of providing an output power greater than 100 microwatts per micron of gate width. The present invention also particularly relates to high-power high-frequency microwave devices, i.e. to microwave transistors capable of providing an output power greater than 100 microwatts per micron of gate width at frequencies above 10 GHz.
The presently most popular microwave transistor technology is MESFET technology. The common features of this technology are that a Schottky-barrier metal is used as the gate in a JFET-like structure. The channel will typically be a lightly doped GaAs semiconductor layer which overlies a semiinsulating substrate. Leakage along the surface from gate to drain is a common problem, and, since reduction of series resistance in the channel is also highly desirable, the gate is commonly recessed. That is, the gate Schottky-barrier metallization is not deposited directly on the surface of the semiconductor, but a recess is etched before the gate is deposited. Preferably the recess is not much wider than the gate, and is etched to (e.g.) one third the depth of the semiconducting layer. This means that the electron population in the channel is slightly removed from the adverse effects of the surface states normally found at the semiconductor surface, and also means that the surface leakage path from gate to drain is longer. However, although this recess etch step is necessary on most MESFET processes. It degrades manufacturability. If the recess etch depth is even slightly nonuniform across a wafer, the pinch-off voltages of the MESFETs on the wafer will vary. This can be disastrous.
Moreover, this technology suffers from several other very important limitations. The output power capability of a MESFET is limited by the gate-drain breakdown voltage and the conduction current through the channel. To improve the breakdown voltage, either a low carrier concentration buffer layer between the gate metal and the channel, or a graded channel approach can be used. See A. Nagashima, S. Umebachi, and G. Kano, IEEE Trans. Electron Devices, vol. ED-25, p537, May 1978, which is hereby incorporated by reference. However, since the breakdown voltage is inversely proportional to the product of the doping level and the active layer thickness, i.e., the channel current, see W. R. Frensley, IEEE Trans. Electron Devices, vol. ED-28, p962, August 1981, and S. H. Wemple, W. C. Niehaus, H. M., Cox, J., V., Dilorenzo, and W. O. Schlosser, IEEE Trans. Electron Devices, vol. ED-27, p1013, June 1980, which is hereby incorporated by reference, the improvement in output power is limited. By employing either an insulating or a semi-insulating buffer layer, the breakdown voltage can be greatly increased due to the much higher breakdown field of the layer, while the current level is maintained. This should result in a device with improved output power. Unfortunately, it has been proved difficult to fabricate metal-insulator-semiconductor or insulated gate FET from III-V compound semiconductors. This is largely due to the large lattice mismatch at the insulator interface and the difficulty in growing a good oxide layer. Even though some attempts have been made in fabricating IGFETs with oxides, see T. Miura and M. Fukuta, IEEE Electron Devices, vol. ED-27, p1147, June 1980, which is hereby incorporated by reference, their suitability for microwave power generation has not been demonstrated. Other workers have attempted the use of Ar ion-implantation for the formation of a semi-insulating gate FET (SIGFET), see B. R. Pruniaux, J. C. North, and A. V. Payer, IEEE Trans. Electron Devices, vol. ED-19, p672, 1972, which is hereby incorporated by reference, and the use of proton bombardment in the source-drain region for the creation of a MIS structure. See H. M. Macksey, D. W. Shaw, and W. R. Wisseman, Electronics Letters vol. 12, p192, 1976, which is hereby incorporated by reference. While the SIGFET approach has resulated in a higher saturated output power, due to the observed higher breakdown voltage, the approach has not been reproducible. It also has inherent higher gate parasitic resistance, which limits its use for high frequency application.
Recently, MIS-type GaAs FETs with AlGaAs as the gate insulator have been reported. See T. J. Drummond et. al., Electronics Letters, vol. 19, p 286, 1983, and Y. Katayama, et. al., Japan. J. of Appl. Phys. vol. 23, p. 150, 1984, which is hereby incorporated by reference. These devices are referred to (among other names) as HEMTs. The reported device structure has a GaAs channel layer which is either undoped or very lightly doped, under a doped AlGaAs layer. This provides very high channel mobilities, but results in very low current levels and high parasitic resistances. These devices were primarily intended for high-speed digital IC applications, and appear to be inherently unsuitable for any application requiring significant power density.
HEMT devices are extremely sensitive to the quality of the interface between the GaAs and AlGaAs layers. In a HEMT structure, the active carrier population is very narrowly confined to a shallow layer underneath this heterojunction. This means that any degradation in the quality of this interface will drastically degrade the device characteristics. This means that HEMTs are difficult to fabricated, and difficult to fabricated reproducibly.
The present invention teaches a heterojunction transistor having an intrinsic (or nearly so) AlGaAs barrier layer over a moderately or heavily doped GaAs channel layer. In the example of this new GaAs power MISFET structure shown in FIG. 1, a highly doped active GaAs layer was used under an undoped Al(0.5)Ga(0.5)As layer to produce enough conduction current through the channel.
This structure has numerous major advantages over the prior art. First, output power is in general proportional to the operating voltage, which is typically limited by gate breakdown voltage. The present invention improves the gate-drain breakdown, which yields higher operating voltage, and therefore higher power from the same size device. A second advantage is that, for devices operating at the same power, it is preferable to operate in a high voltage regime rather than a high current regime, because this simplifies power supply layout. The device of the present invention could be operated at, for example, 15 volts (with the output impedance- transformed if necessary) as opposed to a prior art MESFET power device which would have to be twice as wide and operated at only eight volts.
A third advantage is that cutoff frquency (or extrapolated cutoff frequency FT) is in general proportional to transconductance g-sub-m over gate source capacitance C-sub-gs; the present invention keept transconductance reasonably good while improving the gate to source capacitance significantly (due to the undoped layer of AlGaAs below the gate), and therefore raises the cutoff frequency.
As far as the band diagram difference between the HEMT and the device of the present invention, the inventive device has a band structure as shown in FIG. 2A, where the left side of the band diagram shows the undoped AlGaAs, and in the middle, at the transition from there to the N+ GaAs, the accumulation region is fairly shallow, so the total amount of charge collected in the small well below the AlGaAs level is going to be relatively small as compared to FIG. 2B, which shows the band structure for the HEMT case. In this case the AlGaAs showsn on the left side is N+, and therefore the potential well at the AlGaAs to GaAs boundary is deep and will collect a great many electrons, and therefore the electron distribution in the HEMT is going to be much more two dimensional than in the device of the present invention. A consequence of the more vertically uniform electron distribution in the device of the present invention is that the quality of the GaAs to AlGaAs interface is less critical. Naturally it is nice if you can fabricate the structure with the extremely high quality GaAs to AlGaAs interface which is required for HEMT device, but for the device of the present invention this is not necessary. That means, for example, that metallorganic CVD can be used to fabricate device of the present invention with (optionally) less stringent requirements on interface quality, or other CVD fabrication may be possible. Interface quality is still important in the device of the present invention, but it is not as important as with a HEMT. In the device of the present invention defects at the interface are likely to induce trapped charge which may screen or partially screen the gate signal from the channel, but in the HEMT gate defects are critically important because they may cause scattering effects which directly and immediately degrade the channel mobility. This effect is not as dangerous in the device of the present invention.
A further advantage of the present invention is that the use of dopants in the AlGaAs, as in the HEMT prior art, well also tend to provide some traps, presumably due to deep levels.
A further modified embodiment of the invention, shown in FIG. 4, is achieved by inserting an additional very thin undoped gallium arsenide layer between the undoped AlGaAs layer and the n type gallium arsenide layer. This additional layer might be, for example, 300 angstroms thick, or within the range of 100 to 500 angstroms thick, or, less preferably, thicker or thinner. In this case, a potential will for electrons will exist at the junction between the AlGaAs and the undoped gallium arsenide layers. Other workers have explored using this potential well alone to provide an operating mode analogous to the operation of the HEMT. However, in this embodiment of the invention, not only does this potential well provide an additional channel, but the N type gallium arsenide also provides a channel. Thus, the total channel current is increased, since two separate regions of conduction are both controlled by the gate. The difference in operating characteristics between this embodiment and the first embodiment is in the forward bias operating characteristics. That is, under reverse bias the small potential well between the undoped AlGaAs and the undoped gallium arsenide will be depleted, and the pinch off characteristics of the N type gallium arsenide channel region will predominate. However, when the gate is forward biased (e.g., at voltages between 0 and around 0.6 volts, where the Schottky barrier starts to conduct under forward bias) the small well between the two undoped layers will be in accumulation, and substantial additional current will be obtained at such biases. Thus, the IV curves of FIG. 3 show that the IV characteristics of the FIG. 2C structure are similar to those of the FIG. 2A structure close to pinch off, but under forward bias substantial additional current will flow at the same voltage levels. This alternative embodiment is particularly applicable to integrated circuits combining both enhancement mode and depletion mode transistors.
Two additional important advantages of the present invention are as follows: First, in conventional MESFET art, it is normally necessary to put the gate in a recess, to minimize surface state effects. However, in the present invention, the undoped AlGaAs layer itself serves to minimize surface leakage as discussed above, so that this recess etch step is not necessary. However, in the most preferred embodiment of the present invention, an N+ GaAs layer is used to assist source and drain contact formation. However, this N+ layer is not necessary. For example, after the patterned N+ source/drain implant was applied, the source and drain metallizations could be deposited directly on the doped portions of the AlGaAs layer, or, alternatively, the AlGaAs layer can be etched away selectively at the contact locations.
A processing advantage of the thin N+ gallium arsenide layer which is most preferably used on top of the AlGaAs layer is that it is easy to use an etch which will etch gallium arsenide and stop on AlGaAs. For example, wet etches containing oxidizing agents will accomplish this, as will plasma etches which preferably include some fraction of an oxidizing GaAs. Thus, this N+ layer can easily be patterned, and the timing of the etching step which patterns it is not critical.
A key problem with the prior art on recess etching and conventional gallium arsenide MESFET structures is that the device characteristics are extremely sensitive to the depth of the recess etch, and therefore the recess etch is a critical parameter. However, it is very difficult to control this etch step so that it is absolutely uniform across the slice. This difficulty is completely avoided by the present invention.
According to the present invention there is provided:
A heterojunction device comprising:
A channel layer comprising a first semiconductor material and comprising a dopant concentration of at least ten to the 16th per cubic centimeter; PA2 A barrier layer overlying said channel layer and comprising a second semiconductor material lattice-matched to said first material and having a bandgap wider than the bandgap of said first semiconductor material and comprising a net dopant concentration less than ten to the 16th per cubic centimeter; PA2 First and second source/drain regions electrically connected to said channel layer, and a gate electrode capacitatively coupled to a portion of said channel layer between said source/drain connections.